This invention is directed to commercial or residential integrated heat pump systems that provide water heating, and which can also provide heating or cooling of a comfort zone, as required. The invention is more particularly directed towards an improved control method for delivering water heating from a variable speed heat pump system using outdoor air as the heat source while balancing user comfort and efficiency.
Integrated heat pumps are often employed to provide heating or cooling, as needed, to a residential or commercial comfort zone, i.e., the interior of a residence, office complex, hospital, or the like. Integrated heat pumps are also employed to heat water. A heat pump system for air conditioning, comfort zone heating, and water heating is described in U.S. Pat. No. 4,766,734. Systems of this type can have several modes of operation, such as air conditioning alone, comfort zone space heating alone, water heating alone, air conditioning with water heating, and comfort zone space heating with water heating. Additional modes, such as a defrost cycle, can also be employed. For comfort zone heating and supplemental water heating, resistive elements are employed as auxiliary heating elements for use at times when the heat pump alone cannot produce sufficient heating of the comfort zone or produce enough hot water in the water heater.
For efficient operation, the speed of the variable speed compressor for the heat pump should be controlled in dependence on the outdoor temperature, and on the relation of water temperature to outdoor temperature, as these factors are directly related to the heat load imposed on the heat pump. Moreover, the heat pump should be used only where it is a more efficient heating means then other sources of water heating, and should have its operating conditions limited to safe zones of operation. The heat pump compressor should be controlled to operate only in condition where the motor torque is below a safe limit to torque ceiling. However, no previously proposed heat pump systems have incorporated any means to ensure efficiency and safe operation in this regard.